The present invention relates to a device for managing the displacement of a hydraulic motor having at least two operationally active displacements and able to be connected to two main ducts, for admission and exhaust respectively, the motor comprising two series of ducts each having a first and a second group of distribution ducts, the management device comprising a displacement selector able to occupy at least a large displacement position in which the first groups of distribution ducts of the first and second series are connected to one of the main ducts, while the second groups of distribution ducts of the first and second series are connected to the other main duct, and a small displacement configuration in which the first group of distribution ducts of the first series is connected to one of the main ducts, while the second group of distribution ducts of the first series is connected to the other main duct and a by-pass connection is provided between the first group of distribution ducts of the second series and the second group of distribution ducts of the second series.
The hydraulic motor to which the inventive management device is applied can be used, for example, for the translational movement of a machine or the movement of a tool carried by a machine.
Generally speaking, the speed required for such hydraulic motors is becoming increasingly high, in particular to ensure rapid transfer of the machine between two sites for use of the latter, or for transfer of the tool between two working positions. These motors must therefore be capable of both generating a high torque in order to be able to correctly perform the functions of the machine or of the tool under working conditions, and having a high output speed, for the reasons indicated above.
In order to achieve these two aims, the admission to a hydraulic motor could be provided by a pump of very large dimensions, allowing in particular a high fluid delivery to drive the motor at high speed with a displacement allowing a high output torque to be obtained. However, it would be extremely costly to be forced in this way to over-dimension the pumps and, moreover, it is frequently a requirement that the noise and consumption are minimised for the drive motors of such pumps, which are generally diesel type heat engines or gas engines, something that is at variance with such overdimensioning.
So, it is becoming necessary to allow the hydraulic motor to develop a high torque and to operate at high speed as a function of need, without over-dimensioning the pumps used to supply the motors or the heat engines that drive these pumps.
For these reasons, the hydraulic motor to which the invention is applied has two different displacements, in particular a large displacement developing a high torque for the working of the machine or the tool driven by this motor, and a small displacement allowing operation at higher speed. The constraints mentioned above mean that the small displacement is generally quite significantly reduced in relation to the large displacement, with the motor developing a low torque in this small displacement. In particular, the motor may even have three, or more, displacements, with the smallest displacement being used to ensure a rapid travel of the machine or the tool driven by this motor.
It is known that the braking of such a hydraulic motor can, at least in part, be performed in a hydrostatic fashion, by pressure build-up in the exhaust ducts. The hydrostatic braking efficiency obviously depends on the torque developed by the motor. However, as indicated above, in their smallest displacement, used for rapid travel, the motors develop a relatively low torque, so that there is a risk of the hydrostatic braking efficiency not being guaranteed at low displacement.
Moreover, as has also been indicated above, an effort is made to use, for driving the hydraulic motor feed pump, a less noisy heat engine that is as small as possible. A diesel motor fitted with a turbo-compressor is often used.
During hydrostatic braking, the pressure increases in the return branch of the pump, until it becomes higher than the pressure in the banking up branch, used to feed the motor. As a consequence, the hydraulic motor becomes a pump and the pump becomes a motor, thereby tending to drive the thermal engine which normally serves to drive it. The hydrostatic braking efficiency then depends on the holding torque that can be developed by this heat engine. However, the use of low noise and small dimension heat engines obviously limits the holding torque and the speed of rotation that they are capable of providing. As a consequence, the hydrostatic braking can subject the motor normally used to drive the pump to stresses for which the latter is not designed.
Similarly, and for this same reason, there is a risk of the hydrostatic braking not being properly ensured and/or damage to certain parts of the transmission. In order to limit the risks of damage to the heat engine driving the pump during hydrostatic braking, U.S. Pat. No. 6,338,247 proposes that the admission circuit of the hydraulic motor is fitted with holding valves which are normally open but which partially close when the pressure in the return branch increases too much under the effects of the hydraulic braking. This partial closure causes a loss of load in the return branch and avoids the pump being subjected to excessive pressures and the drive motor of this pump being excessively stressed.
These valves are fitted on the main ducts of the circuit, or at least on the duct that serves as the exhaust for the motor in forward gear. Moreover, if the motor is in small active displacement the braking torque is low and the braking is less efficient. In fact, this system aims to avoid damaging the pump and its drive motor but it does not solve the problem of the inadequate braking at low displacement.
The aim of the invention is to remedy the abovementioned disadvantages by proposing a device for managing the operation of a hydraulic motor that allows, despite the constraints mentioned above, hydrostatic braking to be ensured under favourable conditions, for a simple installation.
This aim is achieved thanks to the fact that the device of the invention comprises restriction means which are able, in the small displacement configuration, to restrict said by-pas connection.
In the small displacement configuration, only the distribution ducts of the first series normally contribute towards delivering the engine torque, since the first and second groups of these ducts are connected to the two different main ducts. So, the small displacement of the motor corresponds to that of the cylinders connected to these distribution ducts of the first series. On the other hand, the first and second groups of distribution ducts of the second series are normally by-passed by being connected together and are therefore at the same pressure such that the corresponding partial displacement is inactive.
According to the invention, in particular when a hydrostatic braking torque in addition to that which would be possible using only the small displacement is necessary, the abovementioned by-pass connection can be restricted. The first and second groups of distribution ducts of the second series are thus placed under different pressures, thereby generating a supplementary torque which is added to the torque generated by this small active displacement and thereby allowing the desired hydrostatic braking torque to be obtained without increasing the holding pressure at the pump. At the same time the motor remains active at small displacement since the first and second groups of main ducts of the first series remain connected, respectively, to each of the main ducts. The total holding torque obtained is therefore higher than that which the small displacement would allow, without restriction of the by-pass connection. The desired hydrostatic braking is therefore obtained without excessive stressing of the motor driving the pump with dissipation of the braking power in the hydraulic motor due to the fact that the by-pass connection is restricted.
The device according to the invention acts selectively on the small displacement, that is to say in the high speed configuration where the need for an additional holding braking torque exists.
It may be decided to integrate the inventive device into the motor, in which case it acts directly at the level of the distribution ducts thereby allowing a compact arrangement.
As a variant, the device of the invention can be external to the motor, acting on the connection between the main ducts of the motor and the main ducts of the circuit. In the latter case the inventive device can even be common to a number of motors, for example two motors on the same vehicle axle.
Advantageously, the device comprises a by-pass selector arranged in the by-pass connection which is able, from an initial position in which it allows a substantially freely open by-pass connection, to be displaced in order to restrict said connection by a restriction.
In its first position the by-pass selector can thereby allow normal functioning at small displacement, while it can be moved in order to restrict the by-pass connection when hydrostatic braking is necessary although the motor is operating at small displacement.
Advantageously, the by-pass connection comprises a first connecting section able to be connected to the first group of distribution ducts of the second series and a second connecting section able to be connected to the second group of distribution ducts of the second series, and the selector comprises a drawer arranged in a bore hole having at least a first and a second opening connected, respectively, with the first and the second connecting sections such that the drawer is able, depending on its position, to establish a substantially free communication or a restricted communication between said sections. These connecting sections may be within the motor, in the event that the inventive device is within the motor. Otherwise, these connecting sections are connected to the respective groups of distribution ducts via the intermediary of the main ducts of the motor to which these distribution ducts are connected.
The selector thus has a simple construction and is easily operated between its various positions.
Advantageously the device comprises a displacement selector which can occupy a large displacement position in which the first groups of distribution ducts are able to communicate together while the second groups of distribution ducts are able to communicate together and occupy a small displacement position in which the first group of distribution ducts of the second series is able to communicate with at least the second group of distribution ducts of the second series. The displacement selector may be internal or external to the motor or even, in the latter case, common to several motors.
Advantageously, said second groups of distribution ducts are able to communicate together in both displacement selector positions.
According to a first embodiment, the displacement selector and the by-pass selector are two different selectors, and in its small displacement position the displacement selector can establish the by-pass connection in which the by-pass selector is arranged.
The displacement selector can be designed and operated in the conventional way while the by-pass selector is operated solely when a restriction of the by-pass connection is necessary, particularly in order to ensure hydrostatic braking at small displacement.
According to another embodiment, the displacement selector and the by-pass selector form a common selector which can occupy a small displacement position with restricted by-pass in which said common selector establishes a restricted communication between the first group of distribution ducts of the second series and the second group of distribution ducts of the second series or between the first group of distribution ducts of the second series and the second groups of distribution ducts of the first and second series.
This common selector has the advantage of allowing an extremely compact design.
Advantageously, the common selector comprises a drawer in a bore hole having at least a first opening connected to the first group of distribution ducts of the first series, a second opening connected to the first group of distribution ducts of the second series and a third opening connected to the second group of distribution ducts of at least the second series; in the large displacement position, said common selector establishes a communication between said first and second openings, isolating these from the third opening; in the small displacement position, the common selector establishes a substantially free communication between the second and third openings isolating these from the first opening and, in the small displacement position with restricted by-pass, the common selector establishes a restricted communication between the second and third openings, isolating these from the first opening.
Advantageously, the displacement position with restricted by-pass is a position of the common selector that is in between the large and small displacement positions of said common selector.
In this case, the design and operation of the common selector can be particularly simple since, from the unrestricted small displacement position in which the by-pass connection is completely free, this connection is restricted in the intermediate position then, as the selector continues its movement, the by-pass connection can disappear in order to ensure large displacement operation.
Advantageously, the restriction of the by-pass connection varies progressively according to the position of the by-pass selector.
The restriction of the by-pass connection can also be in proportion, in particular to the necessary hydrostatic braking torque.
Advantageously, the second groups of distribution ducts of the first and second series are permanently connected together, and the by-pass selector comprises a drawer whose movement control means comprise on the one hand a control chamber connected to the first group of distribution ducts of the second series and on the other a return spring with an antagonistic effect on that of the pressure within said chamber.
So, depending on the calibration of the return spring, the by-pass connection can be automatically restricted when the pressure in the second groups of distribution ducts connected together increases under the effect of hydrostatic braking.
Advantageously, the device comprises a non-return valve able to allow an unrestricted connection between said first and second sections in the direction of circulation of the fluid from the first group of distribution ducts of the second series towards the second groups of distribution ducts.
In particular when the engine has a preferred direction of operation, such a non-return valve is advantageous for allowing short-circuiting of the restriction at small displacement, in the direction of circulation of the fluid corresponding to the non-preferred direction of operation of the motor.
Advantageously, the device has a mechanical brake for the moor and means of operating the motor braking able, when the motor is in small displacement operation, to operate in succession a restriction of the by-pass connection and activation of the mechanical brake.
When it is necessary to brake the machine or tool driven by the motor, for instance, first hydrostatic braking is ensured then, if necessary, additional braking by activation of the mechanical brake.
Advantageously, the device comprises means for operating the braking of the motor that are able, as a function of a braking command and motor operation parameters, to order an initial step of the braking in which the fluid delivery in the main ducts is modified (in particular by being lowered by a reduction in the lower displacement of the pump) and, as a function of the operating situation of the motor at the end of this initial step, to operate at least a second braking step in which the by-pass connection is restricted, allowing in particular limitation of the maximum pressure in the return branch.
Advantageously, the device has means for controlling the position of the by-pass selector that are able, when there is a braking command, to order the movement of said selector so as to control a variable representative of the operation of a vehicle driven by the motor according to a set-point.